This invention relates to a circuit board with at least one electronic component and a method for producing a connection between the circuit board and each component. The connection provides the mechanical fastening of the component onto the circuit board as well as the electrical connection of the component to the circuit board.
Modern electrical appliances, especially measuring instruments, normally have at least one circuit board containing electronic components. These components must be mounted onto the circuit board mechanically and must be electrically connected to wires running to, in or on the circuit board. In order to reduce production costs, surface mountable components, so-called xe2x80x9cSurface Mounted Devices,xe2x80x9d in short xe2x80x9cSMDxe2x80x9d components, are preferably used. SMD components require no circuit board holes for their assembly, rather their contacts are soldered directly onto pads provided on the circuit board. SMD components can be placed on the circuit board mechanically using fitting machines.
Typically in a first step of the manufacturing process for connecting components to a circuit board, soldering paste is applied to the circuit board in all places where components are later to be placed using a screen printing process. Any place where solder has been applied or where a soldering connection exists after the completion of this initial step is called a soldering point.
In the next step, the components are mechanically placed on the circuit board. The circuit board with the components is placed in a furnace. The soldering process is carried out in the furnace, for instance in a controlled protective inert-gas atmosphere, which cycles through a solder-specific temperature cycle.
Alternatively the application of solder to the circuit board and the soldering process can also be carried out using a soldering bath. Since the soldering points are located in between the circuit board and the components, the circuit board must be placed in the soldering bath in such a way that the components project into the bath. In order for the components not to fall into the bath, they are fixed to the circuit board with adhesive before they are placed in the soldering bath. The type of adhesive used, usually a liquid tin alloy, must be able to withstand the temperature of the soldering bath. Such adhesives are commercially available. In order for the adhesive to adhere, it first has to harden after being placed on the circuit board. For this purpose, the adhesives usually have to be heated. Thus, an additional step is required before the soldering process can be carried out, specifically the gluing on of the components to the circuit board. In this case, the adhesive serves exclusively for the mechanical fastening of the components.
Many appliances, such as measuring instruments for measuring pressure, temperature, fluid levels, pressure or flow, are used under conditions in which very high temperatures can occur. Such applications include, for example, the chemistry industry and the food industry. Temperatures of more than 100xc2x0 C. can be used in the food industry during the cleaning and sterilization processes.
One example of a measuring instrument is a fluid level limit switch. Fluid level limit switches are commercially available, and are used to detect when a fluid limit has been reached, as protection against overflow, or as protection against running a pump dry. These limit switches may include a mechanical vibrating structure extending into a receptacle. The mechanical structure is caused to vibrate by means of a piezoelectric element. The resonance frequency and/or the amplitude of the stimulated vibration is measured. Using this information it is determined whether the vibrating structure is vibrating freely or whether it is covered by a fluid material. A fluid level limit switch is described in U.S. patent appl. Ser. No. 09/388,299 filed on Sep. 1, 1999, in which the vibrating structure extending into the receptacle has two vibrating rods which are connected to a membrane. A piezoelectric element is mounted on the side of the membrane facing away from the vibrating rods. The piezoelectric element may, for example, be glued on the membrane. The vibrating rods are excited into vibration through the membrane by the piezoelectric element.
On the side of the membrane facing away from the vibrating rods, the piezoelectric element is electrically and mechanically connected to a flexible circuit board. This circuit board contains electronic components, preferably SMD components, in addition to the piezoelectric element. These SMD components are for instance elements restricting electricity and voltage, or parts of a signal receiver and/or signal processing circuit. Until now the circuit board was first equipped with SMD components in the manufacturing process, the SMD components were soldered, and then the piezoelectric element was connected.
In addition to the aforementioned example, there are a great number of further applications in which other electronic components besides SMD components must be placed on the same circuit board. In these applications it is desirable to mechanically connect all components, preferably in one process, and to connect these components electrically and mechanically with the circuit board in a mass production process.
Soldered connections are only stable at low temperatures, for instance temperatures that lie far below 150xc2x0 C. At temperatures above a solder-specific maximum level, the solder becomes soft and the mechanical and electrical connection is no longer guaranteed. When using piezoelectric elements, this problem becomes evident at lower temperatures as compared to conventional SMD components. This is due to the mechanical vibrations of the piezoelectric element, since the connection is exposed to mechanical stress.
There are adhesives available on the market which make possible connections that still guarantee reliable mechanical connections at higher temperatures, for instance at temperatures of about 150xc2x0 C. Some of these commercially available adhesives are electrically conductive, so that there is also an electric connection when they are used.
These adhesives are normally very watery and must set at the desired gluing point before they initially become sticky, and then finally completely harden, and adhere. These adhesives are therefore not suitable for fast mass production. Transportation and/or the processing of an assembly unit consisting of a circuit board and at least one electric component is not possible until the adhesive has hardened enough for it to at least become sticky.
One objective of the present invention is to provide a circuit board with at least one electric component, and to provide a method for the manufacture of a connection between the circuit board and the component so that fast mechanical mass production is possible and the circuit board can still be used at higher temperatures.
The invention includes a circuit board containing at least one electronic component. The component has at least two electrical contacts, of which the first is initially glued to the circuit board using a conductive adhesive and the second of which is connected to the circuit board by soldering using a solder.
In accordance with one embodiment of the present invention, at least one component is a piezoelectric element.
In accordance with another embodiment of the present invention, the solder is free from lead and contains, in part, silver.
In accordance with another embodiment, the solder is a tin-silver solder.
In accordance with another embodiment, the circuit board is a flexible circuit board, especially one made from polyimide.
In accordance with another embodiment, the two contacts are connected to copper plates on the circuit board.
In accordance with another embodiment, the copper plates contain a nickel/gold connecting metal plating or a layer of tin.
Furthermore, the invention comprises a method for the manufacture of a connection between a circuit board and at least one electronic component, in which:
solder is applied to soldering points and a conductive adhesive is applied to adhesive points, spatially separated from each other, on each surface of the circuit board where a component is to be placed,
the components are placed on the appropriate surfaces,
the circuit board with the components is placed in a furnace and
the furnace is cycled through a temperature cycle, which results in a solder connection between the component part and the circuit board, and through which the adhesive develops its adhesive effect.
According to a further feature of the method, the components are SMD components or components on which all contacts to be connected to a circuit board are in one plane on one side of the component and which form flat surfaces. The components are placed on the circuit board mechanically.
According to one embodiment of the method, the solder is applied mechanically using a screen printing process and the adhesive is applied using a dispenser.